Monday, September 24, 2007

MERGE Statement in SQL Server

Ever had to merge two tables in SQL? Hard work?
The MERGE statement is only available in SQL Server 2008 (and Oracle and DB2 ;) ), but in the meantime - you can mimic it.

If you ever had a large csv file with a new version of data already contained in a table in SQL, you dreamed of something like this, didn't you:

USING (SELECT @id AS id, @name1 AS name1) AS t2 ON =
THEN UPDATE SET t1.name1 = t2.name1
In other words: if the id of a row in the new version matches one in the old version - update the old row; otherwise - insert a new row.

[Of course we're assuming here that you managed to load the contents of the csv file into a staging table].

Well, if you have the luxury of working with SQL Server 2008, the construct above is exactly what you could use there. Have a look here: Using MERGE statement.

Now, since SQL Server 2008 is not yet used in too many commercial projects ;) - it is often the case that you have to implement such functionality on your own.

According to Alex Kuznetsov (MVP):
In SQL Server 2005 you can use OUTPUT clause of an UPDATE statement
in order to achieve that.

See for yourself:
declare @updated_ids table(id int)

update permanent set d=s.d, comment = 'Modified Row'
output into @updated_ids
from permanent p, staging s

insert into permanent
select id, d, 'New Row' from staging where id not in(select id from @updated_ids)
Pretty neat!

You can find Alex's whole post here: Mimicking MERGE Statement in SQL 2005.

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